thanks -- the pointer to epub3 helped. It's apparently something to do with accessability
http://www.idpf.org/accessibility/gu.../epub-type.php
Quote:
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To make ebooks more accessible, you need to consider that many readers will be interacting with the content in non-visual ways, and for that reason the logical reading order must be defined at the markup level. To facilitate this discovery, EPUB 3 includes a new epub:type attribute that allows more precise meanings to be applied to the generic tags, a process called semantic inflection.
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